linux-kernel - Re: [PATCH 2/2] dma-mapping: benchmark: Add padding to ensure uABI remained consistent

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Message-ID: <CAGsJ_4zVR3R7erGP57vxM8vKBARG8BttA=FsCVFjy4QtswcsQw@mail.gmail.com>
Date: Thu, 24 Jul 2025 17:07:08 +0800
From: Barry Song <21cnbao@...il.com>
To: Qinxin Xia <xiaqinxin@...wei.com>, m.szyprowski@...sung.com
Cc: robin.murphy@....com, jonathan.cameron@...wei.com, prime.zeng@...wei.com, 
	fanghao11@...wei.com, linux-kernel@...r.kernel.org, linuxarm@...wei.com, 
	iommu@...ts.linux.dev
Subject: Re: [PATCH 2/2] dma-mapping: benchmark: Add padding to ensure uABI
 remained consistent

On Thu, Jul 24, 2025 at 4:56 PM Qinxin Xia <xiaqinxin@...wei.com> wrote:
>
> The padding field in the structure was previously reserved to
> maintain a stable interface for potential new fields, ensuring
> compatibility with user-space shared data structures.
> However,it was accidentally removed by tiantao in a prior commit,
> which may lead to incompatibility between user space and the kernel.
>
> This patch reinstates the padding to restore the original structure
> layout and preserve compatibility.
>
> Fixes: 8ddde07a3d28 ("dma-mapping: benchmark: extract a common header file for map_benchmark definition")
> Cc: stable@...r.kernel.org
> Acked-by: Barry Song <baohua@...nel.org>
> Signed-off-by: Qinxin Xia <xiaqinxin@...wei.com>

I don’t think these two patches should be part of the same series. This
one is a bug fix and should be handled separately—ideally picked up on
its own and backported to stable.

Also, the subject should not say "Add"—it should be "Restore". I assume
Marek can handle it?

> ---
>  include/linux/map_benchmark.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/linux/map_benchmark.h b/include/linux/map_benchmark.h
> index 62674c83bde4..2ac2fe52f248 100644
> --- a/include/linux/map_benchmark.h
> +++ b/include/linux/map_benchmark.h
> @@ -27,5 +27,6 @@ struct map_benchmark {
>         __u32 dma_dir; /* DMA data direction */
>         __u32 dma_trans_ns; /* time for DMA transmission in ns */
>         __u32 granule;  /* how many PAGE_SIZE will do map/unmap once a time */
> +       __u8 expansion[76];     /* For future use */
>  };
>  #endif /* _KERNEL_DMA_BENCHMARK_H */
> --
> 2.33.0
>

Thanks
Barry